Recent developments of computed x-ray nano-tomography have shown the potential of this technique for 3D physical characterization and failure analysis of 3D-IC interconnects and other features in semiconductor devices. However, current procedures of preparing samples for tomography by milling and cutting the samples with a focused ion beam (FIB) suffer from extremely long procedure times. Focused ion beam sample preparation for tomography applications, such as, for example, X-ray nano CT and Atom Probe Tomography, is typically time consuming as a lot of material removal is involved and the overall process is not very reliable as the success rate is negatively impacted by the poor control of the sample extraction from its bulk environment.
These are the two main problems encountered with state of the art sample preparation of X-ray tomography samples. FIG. 1A is a cross sectional diagraming showing a prior art concept of preparing and cutting of a cylindrical sample from a bulk sample. FIG. 1B shows two perspective images of a sample pillar created with such a technique, the left image with the sample pillar before extraction from the crater, and the right image showing the sample pillar mounted to a sample holder. Referring to FIG. 1A, depicted is a classical symmetric circular geometry FIB milled crater with a two-sided cut. The cut of a pillar having height h and radius Rint is made at around a 45-degree angle from two sides of a circular crater with an outer radius of Rext. The total depth of the crater, z needs to be at least h+Rint to perform the procedure. Such tomography samples are preferred to be cylindrical because a) the tomography is done over 360° rotation and the sample to X-ray source distance needs to be kept minimal (<1 mm) and b) the X-ray absorption length is constant for all sample rotational positions. Preparation of such samples requires the removal of relatively large material volumes for which a high milling rate Plasma-FIB is used. According to prior procedures, the PFIB (plasma focused-ion beam) based preparation method takes approximately 4.5 hours for a cylindrical sample of approximately 100 um diameter and 100 um height, which is too long to be practical if the goal is to prepare many of such samples and obtain X-ray tomography results within 4-8 hours. Approximately 3 hours are needed for PFIB material removal and the other 1.5 hours are used for the cut free, extraction and transfer of the cylindrical sample to a specific sample holder. The latter extraction steps are slow and most importantly not very reliable due to redeposition of material that frequently causes the samples to be damaged or lost during the extraction step. Hence the overall success rate is rather low and slows the average sample preparation time even further.
Not many labs are currently using a (P)FIB for X-ray tomography samples preparation. If not done by FIB, today, tomography sample preparation is done by polishing, sawing and depositing the samples in capillaries. Such methods are very limited, because these cannot be used for every type of materials and can create artefacts such as cracks and delamination. Moreover, unlike FIB, these methods cannot provide an accurate location of the zone of interest within the sample.
Use of a Gallium FIB can avoid creating such artefacts, but a Gallium FIB has maximum milling rates that are not high enough for fabricating tomography samples with large dimensions on the order of approximately 100 μm. Hence, solutions require the use of focused beams with sufficient energy and flux to arrive at acceptable material removal rates. Therefore, because of their high milling rates, plasma-FIB and pulsed lasers are ideal candidates for this kind of sample preparation. What is needed are fast, efficient and accurate methods to control such tools to produce and scan large tomography samples. What is also needed are techniques to improve reliability of sample cut-free and lift-out methods for tomography samples. Still further, methods are needed to improve automation in the sample generation process.